Photovoltaic generators or PV generators for short are used, as part of photovoltaic systems (PV systems), to convert solar energy into electrical energy. In this case, the PV generators usually comprise a plurality of photovoltaic modules (PV modules) each in turn having a multiplicity of photovoltaic cells (PV cells). The PV generators are generally coupled to an inverter which converts the DC voltage generated by the PV generators into an AC voltage in order to feed the latter into a public power grid or into a private power grid (so-called island grid). In this case, in order to increase the voltage, the output voltage of the inverter can be increased to a higher voltage by means of a transformer in order to be able to directly feed into a medium-voltage or high-voltage grid, for example.
Depending on the intended purpose and the amount of the electrical power produced, the structure of such PV systems varies considerably. The AC voltage generated by the inverter can therefore differ, for example, both with regard to its amplitude, its frequency and in the number of phases. Inverters which have a low output power often have a single-phase design and those which output a high power have a three-phase design. However, depending on the design of the grid to which these inverters are connected, other embodiments are also conceivable.
The output voltage of the PV generators is usually between 500 and 1000 V, but there are currently attempts to increase this voltage to 1500 V. Selecting a relatively high DC voltage reduces resistive losses in the DC lines running between the PV generators and the inverters. However, this also results in technical problems, for example with regard to the level of the insulation voltages and, associated with this, the load on the individual components of the PV system. This relates both to components arranged on the DC side and to components arranged on the AC side, for example the transformer or the semiconductor switches of the inverter.
In PV systems in which a transformer is arranged between the AC voltage output of the inverter and the AC grid, the system part from the PV generator to the transformer is initially potential-free. As a result of insulation resistances which are not endlessly high, in particular of DC lines running between the PV generators and the inverters, a potential is established at the positive and negative poles during operation, which potential is approximately symmetrical around ground potential. In the case of a photovoltaic voltage of 1000 V, for example, at the output of a PV generator, the negative pole of the PV generator is at a potential of approximately −500 V with respect to ground potential and the positive pole is at a potential of approximately +500 V with respect to ground potential. Owing to the design, an excessively high negative potential of the PV module or parts of the PV module with respect to ground potential is undesirable in some types of PV modules or an excessively high positive potential of the PV module or parts of the PV module with respect to ground potential is undesirable in other types. Different measures for setting the potentials with respect to ground potential can then be taken. A distinction is made here, for example, between direct (rigid) grounding of a pole of the PV generator, so-called PV+ or PV− grounding, and indirect grounding variants in which the potentials with respect to ground result from (system-related) impedances or from the deliberate introduction of impedances between a pole of the PV generator and ground. In addition, there are also more complex apparatuses which allow targeted potential control. Numerous documents describe corresponding apparatuses which allow the potential of a pole of the PV generator to be shifted. By way of example, the documents DE202006008936 U1, EP2136449 B1 and DE102010060463 B4 are cited here.
Semiconductor switches, for example MOSFETs or IGBTs, are used in the currently customary inverter topologies, for instance the B6 topology. In order to convert a DC voltage into an AC voltage, these semiconductor switches are controlled by means of suitable driver circuits. This is generally referred to as clocking of the semiconductor switches. In this case, very different clocking methods can be used; so-called pulse width modulation methods (PWM methods) are widespread. An insight into these methods is provided, for example, by J. Holtz, “Pulsewidth modulation-A survey,” in Proc. IEEE PESC'92, 1992, p. 11-18 or Kazmierkowski M. P., Dzieniakowski M. A. (1994): Review of current regulation techniques for three-phase PWM inverters. IEEE Conference on Industrial Electronics, Control and Instrumentation, Record, p. 567-575.
In the prior art, the selection of a PWM method suitable for a particular inverter or for a particular PV system is generally based on the fact that an appropriate method contributes, as far as possible, to reducing harmonics and/or minimizing the power loss, for example caused by switching losses. Which method ultimately provides particularly good results depends, inter alia, on the inverter topology. However, other components, for instance snubbers, and their interaction with the PWM method must also be taken into account.
In the known PWM modulation methods, a distinction is made, in particular, between so-called symmetrical and discontinuous or asymmetrical methods. In short, some switches of a bridge are clocked more frequently than others in the asymmetrical methods. In the case of a B6 bridge, the lower semiconductor switches (bottom) are clocked less frequently than the upper semiconductor switches (top) of the inverter bridge in a modulation method, for example. In another method, this relationship is reversed. Such methods are described, for example, in the publication M. RAJENDER REDDY, “SIMPLE AND NOVEL UNIFIED PULSE WIDTH MODULATION ALGORITHM FOR VOLTAGE SOURCE INVERTERS IN THE ENTIRE MODULATION RANGE”, Acta Electrotechnica et Informatica, Vol. 13, No. 3, 2013, p. 48-55, the content of which is therefore fully part of the present disclosure.
SHAOLIANG AN; LU LAI; XIANGDONG SUN; YANRU ZHONG; BIYING REN; QI ZHANG “Neutral point voltage-balanced control method based on discontinuous pulse width modulation for a NPC 3-level inverter”, 2015 9th International Conference on Power Electronics and ECCE Asia (ICPE-ECCE Asia), 2015, p. 2820-2825, discloses a balancing method for the partial voltages in the intermediate circuit of an NPC solar inverter.
US20140 376 293A1 discloses an inverter having a midpoint grounded photovoltaic generator.